Pipette tips for solid phase extraction (SPE) have been developed. Many of these use immobilized sorbent at the narrow end of the tip. The problems with these types of tips include solvent flow rates have to be controlled and the efficiency of the sorbent is not optimal because solution contact with the surface area of the sorbent is limited in fixed beds.
Some pipette extraction products, such as disposable pipette extraction products, including those described in U.S. Pat. No. 6,566,145, use loosely contained sorbent inside of the pipette tip. The sorbent is contained through the use of a screen or filter and an upper porous barrier. Being loosely contained, the sorbent is freely movable and is mixed with solutions providing unsurpassed efficiencies. Additionally, the extractions are rapid as compared to typical extraction methods because conditioning steps and slow flow rates are not required. This type of pipette tip extraction device utilizes dispersive SPE. As used herein, “dispersive” means: the solid phase sorbent may be thoroughly mixed with liquid solutions aspirated into the pipette tip.
While suitable for larger pipette tips, the disposable pipette extraction does not work well with narrow and low volume tips. For example, 5 mL and 1.3 mL disposable pipette extraction tips work well for extracting drugs from urine or pesticides from fruit and vegetable extracts. However, the disposable pipette extraction method is irreproducible when incorporated in 1 mL pipette tips that are used for 96 well plates and robotics. The poor reproducibility is mainly a result of inefficient mixing of the sorbent material with the sample solution. The sorbent bed after conditioning tends to adhere to the walls of the narrow tip, and consistent mixing with the sample solution is therefore difficult to achieve. Oftentimes, the sorbent bed floats on top of the sample solution rather than interacting with the solution, and irreproducibility occurs.
To overcome this problem, one device, known commercially by the name ASPIRE®, incorporates an intermediate porous barrier. The intermediate porous barrier ensures the sorbent is mixed with the solutions by aspirating and dispensing the solutions through the intermediate barrier. The intermediate barrier allows liquid solutions to pass through, but prevents the sorbent from passing through. However, a drawback to this type of extraction tip is that the intermediate barrier may cause losses in recovery because some of the liquid solution will inevitably get trapped by the porous membrane. Another drawback is that the existence of this barrier creates back pressure issues.
Accordingly, there exists a need for a dispersive SPE device that may be used to process liquid sample solutions for reproducible chemical analysis with low back pressure.